Short-wave amplifying system



Jan. 13, 1931. R. A. HElslNG SHORT WAVE AMPLIEYING SYSTEM Filed Jan. 2,1929 @VWF A TTOR/VEY Patented Jan. 1'3, 1931 UNITE-D STATES PATENTOFFICE vRAYMOND A. HEISING, 0F MILLBURN, NEW JERSEY, ASSIGNOR TO BELLTELEPHONE LABORATORIES, INCORPORATED, OIF NEW YORK, N. Y., A CORPORATIONOF NEW YORK SHORT-WAVE AMPLIFYING SYSTEM Application filed January 2,1929. YSerial No. 329,881.

Reactive coupling between elements of a vacuum tube or portions of thecircuit give rise to such well-known effects as regeneration, loss ofamplification or degeneration, distortion and, in extreme cases, theproduction of actual oscillations or singing Among the principalundesired couplings in such a circuit may be mentioned the inherentcapacity between electrodes ofthe tube, especially that between theplate and grid; inductive coupling between windings in the input andoutput circuits of the tube, and reactance in the lcad wires orbetween/lead wires connecting portions of the circuit.

Various methods have been devised for reducing the harmful effects ofthe inherent capacity, such as: 1. The use of circuit ele,- nientsexternal to the tube for developing current or voltage componentstending to counteract those arisinggfrom the inherent capacity; and 2.The use of an internal shield or screen to reduce the inherent capacity.

The effects of inductive coupling have been guarded against bv shieldingand in some cases disposing the leads or windings so as to reduce themutual inductance between certain portions of the circuit.

It has been recognized that the difficulty of securing protectionagainst undesired coupling increases enormously at higher and higherfrequencies. For example, shielding, anti-singing connections or othercapacity neutralizing means that suiiice in the range of frequenciesemployed for radio broadcast may prove useless or inadequate atfrequencies twenty oi' thirty times higher. While the invention is notlimited in its use to any range of frequencies, it provides effectiveprotection against undesired coupling at the eXtreme high frequenciesutilizable in radio signaling,

' even higher.

Applicant has found that a particularly troublesome coupling in a vacuumtube circuit, especially at very high frequencies, is

the coupling which arises from the use of a common lead for l connectingthe grid and plate circuits to an element of the tube. Such a lead maybe the common connection to the cathode. In the case of a screen-gridtube, another common lead which introduces detrimental coupling effectsis the lead from the screen to the plate and grid. circuits. So long asthese common leads are present and contain appreciable inductance, noamount of shielding can eliminate the coupling between the input andoutput circuits of the tube.

Applicant has devised as one feature of the invention, a circuit whichreduces the eects of common leads to an innocuous degree even at theextreme high frequencies corresponding to the 'so-called short-waverange used in radio signaling.

Another feature of the invention relates to a novel type of shieldingespecially effective in preventing detrimental coupling between elementsof a vacuum tube circuit.

The invention in a preferred form. contemplates a practically completeelimination of all harmful coupling eEects in a vacuum tube circuit andto accomplish this combines reduction of the inherent capacity betweenelements of the vacuum tube, reduction of the common lead eiects, andisolation of .input and output circuit elements by proper shield ing.The actual type of means for reducing inherent tube capacity maycomprise a screen-grid construction but the invention is not limited tosuch means since. other capacity-reducing or capacity-annulling meansmay be used. Features of the invention, as noted above, relate toparticular connections or constructions for reducing the common leadeffects and for shielding, and will be claimed herein in addition to thegeneral combination referred to above as the preferred form whichthe-invention may take.

In the drawings Fig. l represents a prior art circuit which is disclosedhere for the i by lead wires to an input and an output circuitcontaining respectively the tuned circuits 8 and 9 which, it will beassumed, may be tuned to any desired frequencies.

The connection'of the filament to the grid and plate circuits is madethrough the wire 6 to one of the points 10 and theconnection of thescreen is through conductor 7 to one of the points 10. It is obviousthat the plate current flows in a circuit including elements 2 and 3within the tube and the external circuit 9, 10 'and 6. (For simplicitybatteries have been omitted in this ligure.) The. grid 4 is connectedthrough the external tuned circuit 8 to the point 10 and thence throughcon-` ductor 6 to the filament so that any drop of potential inconductor 6 caused, forexample, by the flow of space current is appliedas a volta e to the grid 4. The conductor 6 therefore orms a couplingelement between the plate and grid circuits, since it is common to bothcircuits.

In a similar manner conductor 7 forms a coupling element between theplate and grid circuits. For example the screen 5 forms one plate of acapacity, another plate of which is the anode 3. Charging current forthis capacity 'lows in the path 5, 3, 9, 10, 7. Screen 5 also hascapacity with respect to the grid 4, so that charging current for thiscapacity flowsin the path 5, 4, 8, 10, 7. A drop of potential inconductor 7 therefore produced by the How of charging current due to thecapacity 5-3 is imparted to the rid 4.

Ordinarily the reactive impedance o leads 6 and 7 would be negligiblysmall at speech frequencies and at frequencies considerably higher. Atextremely high frequencies, however, the impedance of lead 6 or 7becomes appreciable and applicant has found that with any ordinaryconstruction employed, a common lead 6 or 7 between the plate and gridcircuits shows sufficient impedance in the common lead 6 or 7 at veryhigh frequencies to produce a noticeable amount of retro-active effectbetween the plate and grid circuits.

With the types of tubes ordinarily used the retro-active e ects due tocapacity between the grid 4 and plate 3 would, in the absence of someprotective means, such as the screen 5 predominate over the retro-activeeffects due to the common lead 6. If the retro-active Vor eliminatingtheeifects of coupling due to the common lead 6 in connection with ameans for substantially eliminating the` grid plate capacity. Where thelatter means comprises the screen 5 the invention further-comprisesv ameans for eliminating or reducing the effects of coupling produced inthe common lead 7.

The common lead 6 is in series with the plate impedance of the tubewhich is ordinarily high and it has been found that it is less importantto eliminate the effects of coupling'due to the common lead 6 than thatdue to the common lead 7, since the shield 5 presents very much lowerimpedances toward both the plate and the grid than the impedance betweenthe plate 3 and filament 2 of the tube. V

Referring now to Fig. 2 a circuit in accordance with the invention ishere shown for reducing the retro-active effects due to the coupling ofthe common lead 7 of Fig. 1. In Fig. 2 a tube construction is shownsomewhat diil'erent from that of Fig. 1, but con.

taining the same elements, that is, cathode 2, anode 3, grid 4 andscreen or shield 5. The lament or cathode 2 has its leads extendedthrough opposite walls of the envelope 1 and connected to filamentheating battery 11, one pole of which is shown connected by common lead6 to a ground plate 12'which may be a wire conductor, but in thepreferred construction where shielding is used this element would beconstituted by a portion of the shield. The plate 3 is connected by alead passing through the right-hand wall of the envelope 1 andconnecting to the output tuned circuit 9, thence through plate battery13 shunted by condenser l14 to ground plate 12. The Grid 4 is connectedby a lead passing through the opposite wall of the envelope 1 to theinput tuned circuit 8 and through a grid biasing battery 15 to groundplate 12. Battery 15 may be shunted by condenser 16. The screen 5 isprovided with two connecting leads passing through op-l posite walls ofthe tube. The lead 21 at the left passes through condenser 17 to groundplate 12 and lead 20 at the right is connecled throu h battery 18shunted by condenser 19, and t ence to ground plate 12. Battery 18serves to place a suitable polarizing voltage on the screen, whilecapacity 17 ac-'s as a blocking condenser for the direct current frombattery 18.

Capacity exists between the anode 3 and the screen 5 and between thegrid 4 and the screen 5 as described in connection with Fig. 1. However,in the circuit of Fig. 2, charging current for the capacity 3-5 passesprincipally through the conductor 20, plate 12, condenser 14 and tunedcircuit 9, whereas charging current forrthe capacity 4-5 passes throughconductor 21, condensers17 and 16 and tuned circuit 8. The two circuitsfollowed by the charging currents of therespective capacities 3, 5 and4, 5 are therefore separated in the circuit of Fig. 2 and madeindividual respectively to the plate circuit and grid circuit. In anac.ual practical elnbodiment, the circuit 8 is associated with, the gridas intimately as convenient. Conductor 21 is therefore made as short aspossible. Sometimes condenser 17 is located as near to the. tubeenvelope as possible so as to make the connection between condenser 17-and shield 5 as short as possible, and the connection from battery 15,indicated at 70, is made with the lower end of condenser 17 (at point71) instead of to the point shown. Condenser 19 and connection 72 aretreated likewise. In this manner the impedance of the" connecting leadsis kept to the smallest practicable value, so that practically all ofthe charging current between the screen 5 and the anode 3 is confined tothe output circuit, While that due to the capacity between screen 5 andgrid 4 is practically confined to the grid circuit.

Although conductors 20 and 21 are to a certain extent in parallel due tobeing connected by the ground plate or conductor 12, the impedance ofconductor 12 Will ordinarily at very high frequencies be high enough tobe influential in making the grid circuit and plate circuit chargingcurrents remain confined Within their own circuit branches orcompartments. This is further emphasized by the skin effect actionproduced When two frequency circuits are in parallel, which causes themajor part of the current to flow in the section that is nearest theloop circuit in which they are connected and to have little relation tothe impedances of the two branches themselves. In the circuit asdisclosed in this figure no attempt has been made to elimina' e theeffect of common lead 6, since as explained above the effect of thiscommon lead is less than that produced by a common lead for the screenWhich is avoided in this figure.

ln Fig. 3 a circuit is disclosed for reducing the common lead effect ofboth the cathode lead and a lead to the screen or shield. In this figurethe various elements correspond to those bearing the same referencecharacters in Fig. 2. In this figure the screen 5 has leads projectingfrom opposite walls of the tube and connected as in Fig. 2 to the groundplate 12. Also the filament 2 ha-s leads extending through oppositeWalls of the tube and connected by leads 24 and 25 to ground plate 12.The filament heating battery 11 is shunted by capacity 23. As described1n connection with conductors 20I and 21 of Fig. 2

the conductors 24 and 25 provide separate wvhereas the grid isconn-ected to the cathode through conductor-24. The drop of highfrequency potential produced in lead 24 is smaller than that occurringin lead 25, so that less of this potential is impressed on the grid thanwouldbe the case if the grid Were connected as in Fig. 2. Thisconstruction therefore reduces the effects of coupling due to the commonlead to the cathode, as Well as that due to the common lead for thescreen.

Referring noW to Fig. 4 a construction is here shown involvingelimination of feed back due to coupling through common leads andemploying a novel type of shielding in connection with vacuum tubesaccording to the invention.

The shield 30 is divided into compartments 31, 32, 33 and 34 andcontains a circuit constructed according to the invention comprising tWostages of radio frequency amplification and a detector. The radiofrequency stages comprise tubes 35 and 36, While the detector stageincludes tube 37. Each of these tubes projects through an aperture in apartition forming part of the shielded enclosure.

-The type of circuit employed in connection with each tube is generallythe same as that shown in Fig. 3 and the corresponding parts may beidentified by the use of the same reference characters. The filaments ofall three tubes are supplied With heating current in parallel frombattery 11 through respective high frequency choke coils 39, 40 and 41.The anodes are supplied in parallel from plate battery 13 throughrespective high frequency choke coils 43, 44 and the au-A dio circuit51, 52, 53. Potential for the screen of each tube is derived frombattery 18, the screen of each tube being connected through a separatehigh frequencyfchoke coil 47, 48, 49 respectively. The audio part of thecircuit, a-s shown diagrammatically, includes a suitable audio-amplifier52 and a suitable receiver 55.

Referring more particularly to the type of shielding employed, it willbe noted that compartment 31 includes the grid circuit of the firststage of amplification. Compartment 32 contains the output circuit ofthe first stage and the input circuit of the second stage. Compartment33 similarly contains the output circuit of the second stage and theinput circuit of the detector, the output of Which is contained incompartment 34. The shield as a Whole is grounded at 60 and eachpartition is intimately associated with and eX- tends completely aroundeach tube. These partitions are so placed as e'ectively to isolate allof the input wiring from all of the output wiring in the case of eachstage. The partition may be placed mid-way of the tube as in the case oftube 35 or nearer to one or the other end of the tube (as far as theshield extends) as the case of tubes 36 and 37, these threepositions-'of the shield being" indicated for purposes of illustration.Any of these positions is suitable, the principal requirement beine`that the plane of the shield cuts through the tube at such a point thatsubstantially all of the input leads are on one side of it, Whilesubstantially all of the output leads are on the opposite side. Theshield and tube construction should be such that the shield approachesthe tube elements or tube leads as intimately as possible Withoutproducing undue shunting effects to ground by capacity with respect toany of the elements or conductors.

The input of the first stage of amplification is adjustably coupled toan antenna circuit 61 for receiving radio Waves.

The operation of the circuit of Fig. 4 will be apparent from thedescription already given, particularly in connection with the previousfigures. Radio Waves incident on antenna 61 are transmitted into tunedcircuit 8 and applied to the grid of the tube 35. The grid circuit ofthis stage is traceable from the grid through tuned circuit 8, grid biasbattery 15, a portion of shield 30 and conductor 24 to the cathode. Theamplified signal variations appearing in the plate circuit flow throughthe path from the anode, output tuned circuit 9, condenser 14, a portionof shield 30 and condenser 23 to the cathode.

arate from and isolated from the plate circuit, except for suchassociation as exists between the tube elements themselves.

The inherent capacity of tube 35g`which if not annulled or reduced'would result in undesired feed back especially at short Waves isreduced by screen 5 which preferably surrounds ythe anode 3. Screen 5has a screento-grid connection Wholly contained Within shieldedcompartment 31, this connection extending through conductor 21,condenser 17, battery 15 and tuned circuit 8 to the grid. Screen 5 alsohas a plate-to-screen connection wholly contained Within shieldedcompartment 32, this circuit including conductor 20, condensers 19 and14 and tuned circuit 9 to the plate.

Elimination of retro-active effects is thus realized Which might arisefrom three causes. First, that which would arise from inherent tubecapacity is eliminated by the shield 5; second, that which might arisefrom the use of a common lead for the screen is avoided by the use ofindividual grid and plate leads The grid circuit is thus seen to. becontained in a shielded compartment sepcontained in se arate shieldedcompartments; and thir that which would arise from the use of a commonfilament lead is eliminated by the use of individual grid and platefilament leads contained in separate shielded compartments. 39 and 47confine the high frequency currents to the shielded enclosure, Whilepermitting direct current to be applied to the tube elements.

The amplified signal in the output of stage 35 is applied to the grid ofstage 36 through series condenser 62. Grid leak resistance 63 connectsthe grid of this tube with the filament through suitable biasing meansas desired. This stage is in all essential respects similar to stage 35and needs no-urther description. The amplified Wave from stage 36 isapplied across anti-resonant circuit 69, which is similar to tunedcircuit 9, and is applied to the grid of the detector through seriescondenser 63. The detector grid circuit is connected to the filamentthrough choke coil 64 and suitable biasing means if desired. Detector 37is connected to the audio amplifier 52 through series inductances 51 and53 so that the signal may be further amplified at 52 and supplied to theloud speaker 55. Inductances 51, 53 and suitable shunt capacities forma'low pass filter pergnitting the signal to pass freely to amplifier Itwill be understood, of course, that the invention is not limited to anyparticular number of stages or to the specific embodiments that havebeen shown for purposes of illustration, but that the invention issusceptible of various modifications within the scope of the appendedclaims.

What is claimed is:

1. In combination, a vacuum tube containing a cathode, an anode and agrid or the equivalent, a grid lead and a cathode lead extending throughone Wall of the tube, an anode lead and a cathode lead extending throughthe opposite Wall of the tube, input and output circuits connected tosaid pairs of leads respectively, and means to reduce the inherentgrid-anode capacity in said tube.

2. In combination, a vacuum tube for the transmission of electricalwaves containing a cathode, an anode, a grid and a screen, said tubehaving individual anode-to-screen and id-to-screen circuits external tosaid tube, substantially electrically independent of each other asregards the transmitted Waves.

y 3. A space discharge device having cathode. grid, anode and screengrid elements, an external shield forming compartments, an alternatingcurrent connection from the cathode to said shield, a connection whollyWithin one compartment of the shield, be-

tween the grid and the shield, a connectionv wholly within the othercompartment be- Choke coils A' including` cathode. grid, anode tween theanode and the shield, and a separate connection in each compartmentbetween the screen grid and the shield.

4. In combination,

equivalent and a shield member within the enclosing envelope, anexternal shield forming compartmentsinto which said device projects. anexternal grid-cathode circuit lying wholly Within one of saidcompartments, an external anode-cathode circuit lying wholly withinanother of said compartments, and a separate shield-member-to-cathodeconnection in each compartment'.

5. In a vacuum tube system, an envelope and screengrid elements,grid-cathode and anode-cathode leads extending through opposite walls ofsaid envelope respectively` a lead through each of said walls for saidscreen-grid, and separate external circuit connections between anode andscreen grid, and grid and screen grid, resnectivelv utilizing adifferent one of said leads to said screen grid.

6. A receiving circuit -for short waves comprising a plurality ofamplifier stages each including` cathode, grid, anode and screen-gridelectrodes, and means for reduc ing the coupling eiiects of a commonlead from the screen-grid to the grid and anode,

comprising, separate eXternalscreen-grid to anode and screen-gr1d togrld connectlons extending in different directions through the tubewalls and enclosed in different shielded compart1nents.

In witness whereof, I hereunto subscribe my name this 31st day ofDecember, 1928. f RAYMOND A. HEISING.

a space discharge dec vvice having a cathode, an anode, a grid or

